============

Basic Python

============

This document is intended to be handed out at the end of the workshop. It has

been designed for Engineering students who are Python beginners and have basic

programming skills. The focus is on basic numerics and plotting using Python.

The system requirements:

  * Python - version 2.5.x or newer.

  * IPython

  * Text editor - scite, vim, emacs or whatever you are comfortable with.

Introduction

============

The Python programming language was created by a dutch named Guido van Rossum.

The idea of Python was conceived in December 1989. The name Python has nothing

to do with the reptilian, but its been named after the 70s comedy series 

"Monty Python's Flying Circus", since it happens to be Guido's favourite 

TV series. 

Current stable version of Python is 2.6.x, although Python 3.0 is also the stable

version, it is not backwards compatible with the previous versions and is hence

not entirely popular at the moment. This material will focus on the 2.6.x series.

Python is licensed under the Python Software Foundation License (PSF License) 

which is GPL compatible Free Software license (excepting license version 1.6 and 2.0)

It is a no strings attached license, which means the source code is free to modify

and redistribute.

The Python docs define Python as "Python is an interpreted, object-oriented, 

high-level programming language with dynamic semantics." A more detailed summary

can be found at http://www.python.org/doc/essays/blurb.html. Python is a language that

has been designed to help the programmer concentrate on solving the problem at hand

and not worry about the programming language idiosyncrasies.

Python is a highly cross platform compatible language on account of it being an 

interpreted language. It is highly scalable and hence has been adapted to run on 

the Nokia 60 series phones. Python has been designed to be readable and easy to use

**Resources available for reference**

* Web: http://www.python.org

* Doc: http://www.python.org/doc

* Free Tutorials:

    * Official Python Tutorial: http://docs.python.org/tut/tut.html

    * Byte of Python: http://www.byteofpython.info/

    * Dive into Python: http://diveintopython.org/

**Advantages of Python - Why Python??**

* Python has been designed for readability and ease of use. Its been designed in 

  such a fashion that it imposes readability on the programmer. Python does away

  with the braces and the semicolons and instead implements code blocks based on 

  indentation, thus enhancing readability. 

* Python is a high level, interpreted, modular and object oriented language.

  Python performs memory management on its own, thus the programmer need not bother

  about allocating and deallocating memory to variables. Python provides extensibility

  by providing modules which can be easily imported similar to headers in C and 

  packages in Java. Python is object oriented and hence provides all the object oriented

  characteristics such as inheritance, encapsulation and polymorphism.

* Python offers a highly powerful interactive programming interface in the form

  of the 'Interactive Interpreter' which will be discussed in more detail in the 

  following sections.

* Python provides a rich standard library and an extensive set of modules. The 

  power of Python modules can be seen in this slightly exaggerated cartoon

  http://xkcd.com/353/

* Python interfaces well with most other programming languages such as C, C++ 

  and FORTRAN.

Although, Python has one setback. Python is not fast as some of the compiled 

languages like C or C++. Yet, the amount of flexibility and power more than make

up for this setback.

The Python Interpreter

======================

The Interactive Interpreter

~~~~~~~~~~~~~~~~~~~~~~~~~~~

Typing *python* at the shell prompt on any standard Unix/Gnu-Linux system and

hitting the enter key fires up the Python 'Interactive Interpreter'. The Python

interpreter is one of the most integral features of Python. The prompt obtained

when the interactive interpreter is similar to what is shown below. The exact

appearance might differ based on the version of Python being used. The ``>>>``

thing shown is the python prompt. When something is typed at the prompt and the

enter key is hit, the python interpreter interprets the command entered and

performs the appropriate action. All the examples presented in this document are

to be tried hands on, on the interactive interpreter.

::

  Python 2.5.2 (r252:60911, Oct  5 2008, 19:24:49) 

  [GCC 4.3.2] on linux2

  Type "help", "copyright", "credits" or "license" for more information.

  >>> 

Lets try with an example, type ``print 'Hello, World!'`` at the prompt and hit

the enter key. 

::

  >>> print 'Hello, World!'

  Hello, World!

This example was quite straight forward, and thus we have written our first

line of Python code. Now let us try typing something arbitrary at the prompt.

For example: 

::

  >>> arbit word

    File "<stdin>", line 1

      arbit word

              ^

  SyntaxError: invalid syntax

  >>>

The interpreter gave an error message saying that 'arbit word' was invalid

syntax which is valid. The interpreter is an amazing tool when learning to

program in Python. The interpreter provides a help function that provides the

necessary documentation regarding all Python syntax, constructs, modules and

objects. Typing *help()* at the prompt gives the following output:

::

  >>> help()

  Welcome to Python 2.5!  This is the online help utility.

  If this is your first time using Python, you should definitely check out

  the tutorial on the Internet at http://www.python.org/doc/tut/.

  Enter the name of any module, keyword, or topic to get help on writing

  Python programs and using Python modules.  To quit this help utility and

  return to the interpreter, just type "quit".

  To get a list of available modules, keywords, or topics, type "modules",

  "keywords", or "topics".  Each module also comes with a one-line summary

  of what it does; to list the modules whose summaries contain a given word

  such as "spam", type "modules spam".

  help> 

As mentioned in the output, entering the name of any module, keyword or topic

will provide the documentation and help regarding the same through the online

help utility. Pressing *Ctrl+d* exits the help prompt and returns to the

python prompt. 

Let us now try a few examples at the python interpreter. 

Eg 1:

::

  >>> print 'Hello, python!'

  Hello, python!

  >>>

Eg 2:

::

  >>> print 4321*567890

  2453852690

  >>> 

Eg 3:

::

  >>> 4321*567890

  2453852690L

  >>>

::

  Note: Notice the 'L' at the end of the output. The 'L' signifies that the

  output of the operation is of type *long*. It was absent in the previous

  example because we used the print statement. This is because *print* formats

  the output before displaying.

Eg 4:

::

  >>> big = 12345678901234567890 ** 3

  >>> print big

  1881676372353657772490265749424677022198701224860897069000

  >>> 

::

  This example is to show that unlike in C or C++ there is no limit on the

  value of an integer.

Try this on the interactive interpreter:

``import this``

*Hint: The output gives an idea of Power of Python*

*ipython* - An enhanced interactive Python interpreter

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The power and the importance of the interactive interpreter was the highlight

of the previous section. This section provides insight into the enhanced

interpreter with more advanced set of features called **ipython**. Entering

*ipython* at the shell prompt fires up the interactive interpreter. 

::

  $ ipython

  Python 2.5.2 (r252:60911, Oct  5 2008, 19:24:49) 

  Type "copyright", "credits" or "license" for more information.

  IPython 0.8.4 -- An enhanced Interactive Python.

  ?         -> Introduction and overview of IPython's features.

  %quickref -> Quick reference.

  help      -> Python's own help system.

  object?   -> Details about 'object'. ?object also works, ?? prints more.

  In [1]: 

This is the output obtained upon firing ipython. The exact appearance may

change based on the Python version installed. The following are some of the

various features provided by **ipython**:

    Suggestions - ipython provides suggestions of the possible methods and

    operations available for the given python object.

Eg 5:

::

  In [4]: a = 6

  In [5]: a.

  a.__abs__           a.__divmod__        a.__index__         a.__neg__          a.__rand__          a.__rmod__          a.__rxor__

  a.__add__           a.__doc__           a.__init__          a.__new__          a.__rdiv__          a.__rmul__          a.__setattr__

  a.__and__           a.__float__         a.__int__           a.__nonzero__      a.__rdivmod__       a.__ror__           a.__str__

  a.__class__         a.__floordiv__      a.__invert__        a.__oct__          a.__reduce__        a.__rpow__          a.__sub__

  a.__cmp__           a.__getattribute__  a.__long__          a.__or__           a.__reduce_ex__     a.__rrshift__       a.__truediv__

  a.__coerce__        a.__getnewargs__    a.__lshift__        a.__pos__          a.__repr__          a.__rshift__        a.__xor__

  a.__delattr__       a.__hash__          a.__mod__           a.__pow__          a.__rfloordiv__     a.__rsub__          

  a.__div__           a.__hex__           a.__mul__           a.__radd__         a.__rlshift__       a.__rtruediv__      

In this example, we initialized 'a' (a variable - a concept that will be

discussed in the subsequent sections.) to 6. In the next line when the *tab* key

is pressed after typing '*a.*' ipython displays the set of all possible methods

that are applicable on the object 'a' (an integer in this context). Ipython

provides many such datatype specific features which will be presented in the

further sections as and when the datatypes are introduced.

Editing and running a python file

=================================

The previous sections focused on the use of the interpreter to run python code.

While the interpeter is an excellent tool to test simple solutions and

experiment with small code snippets, its main disadvantage is that everything

written in the interpreter is lost once its quit. Most of the times a program is 

used by people other than the author. So the programs have to be available in 

some form suitable for distribution, and hence they are written in files. This 

section will focus on editing and running python files. Start by opening a text 

editor ( it is recommended you choose one from the list at the top of this page ).

In the editor type down python code and save the file with an extension **.py** 

(python files have an extension of .py). Once done with the editing, save the 

file and exit the editor. 

Let us look at a simple example of calculating the gcd of 2 numbers using Python:

**Creating the first python script(file)**

::

  $ emacs gcd.py

    def gcd(x,y):

      if x % y == 0:

        return y

      return gcd(y, x%y)

    print gcd(72, 92)

To run the script, open the shell prompt, navigate to the directory that 

contains the python file and run ``python <filename.py>`` at the prompt ( in this 

case filename is gcd.py )

**Running the python script**

::

  $ python gcd.py

  4

  $ 

Another method to run a python script would be to include the line

``#! /usr/bin/python``

at the beginning of the python file and then make the file executable by 

$ chmod a+x *filename.py*

Once this is done, the script can be run as a standalone program as follows:

$ ./*filename.py*

Basic Datatypes and operators in Python

=======================================

Python provides the following set of basic datatypes.

  * Numbers: int, float, long, complex

  * Strings

  * Boolean

Numbers

~~~~~~~

Numbers were introduced in the examples presented in the interactive interpreter

section. Numbers include types as mentioned earlier viz., int (integers), float 

(floating point numbers), long (large integers), complex (complex numbers with 

real and imaginary parts). Python is not a strongly typed language, which means 

the type of a variable need not mentioned during its initialization. Let us look

at a few examples.

Eg 6:

::

  >>> a = 1 #here a is an integer variable

Eg 7:

::

  >>> lng = 122333444455555666666777777788888888999999999 #here lng is a variable of type long

  >>> lng

  122333444455555666666777777788888888999999999L #notice the trailing 'L'

  >>> print lng

  122333444455555666666777777788888888999999999 #notice the absence of the trailing 'L'

  >>> lng+1

  122333444455555666666777777788888889000000000L

Long numbers are the same as integers in almost all aspects. They can be used in

operations just like integers and along with integers without any distinction.

The only distinction comes during type checking (which is not a healthy practice).

Long numbers are tucked with a trailing 'L' just to signify that they are long.

Notice that in the example just lng at the prompt displays the value of the variable

with the 'L' whereas ``print lng`` displays without the 'L'. This is because print 

formats the output before printing. Also in the example, notice that adding an 

integer to a long does not give any errors and the result is as expected. So for

all practical purposes longs can be treated as ints.

Eg 8:

::

  >>> fl = 3.14159 #fl is a float variable

  >>> e = 1.234e-4 #e is also a float variable, specified in the exponential form

  >>> a = 1

  >>> b = 2

  >>> a/b #integer division

  0

  >>> a/fl #floating point division

  0.31831015504887655

  >>> e/fl

  3.9279473133031364e-05

Floating point numbers, simply called floats are real numbers with a decimal point.

The example above shows the initialization of a float variable. Shown also in this

example is the difference between integer division and floating point division.

'a' and 'b' here are integer variables and hence the division gives 0 as the quotient.

When either of the operands is a float, the operation is a floating point division,

and the result is also a float as illustrated.

Eg 9:

::

  >>> cplx = 3 + 4j #cplx is a complex variable

  >>> cplx

  (3+4j)

  >>> print cplx.real #prints the real part of the complex number

  3.0

  >>> print cplx.imag #prints the imaginary part of the complex number

  4.0

  >>> print cplx*fl  #multiplies the real and imag parts of the complex number with the multiplier

  (9.42477+12.56636j)

  >>> abs(cplx) #returns the absolute value of the complex number

  5.0

Python provides a datatype for complex numbers. Complex numbers are initialized 

as shown in the example above. The *real* and *imag* operators return the real and

imaginary parts of the complex number as shown. The *abs()* returns the absolute

value of the complex number.

Variables

~~~~~~~~~

Variables are just names that represent a value. Variables have already been 

introduced in the various examples from the previous sections. Certain rules about

using variables:

  * Variables have to be initialized or assigned a value before being used.

  * Variable names can consist of letters, digits and underscores(_).

  * Variable names cannot begin with digits, but can contain digits in them.

In reference to the previous section examples, 'a', 'b', 'lng', 'fl', 'e' and 'cplx'

are all variables of various datatypes.

::

  Note: Python is not a strongly typed language and hence an integer variable can at a

  later stage be used as a float variable as well.

Strings

~~~~~~~

Strings are one of the essential data structures of any programming language.

The ``print "Hello, World!"`` program was introduced in the earlier section, and

the *"Hello, World!"* in the print statement is a string. A string is basically 

a set of characters. Strings can be represented in various ways shown below:

::

  s = 'this is a string'              # a string variable can be represented using single quotes

  s = 'This one has "quotes" inside!' # The string can have quotes inside it as shown

  s = "I have 'single-quotes' inside!"

  l = "A string spanning many lines\

  one more line\

  yet another"                        # a string can span more than a single line.

  t = """A triple quoted string does  # another way of representing multiline strings.

  not need to be escaped at the end and

  "can have nested quotes" etc."""

Try the following on the interpreter:

``s = 'this is a string with 'quotes' of similar kind'``

**Exercise: How to use single quotes within single quotes in a string as shown 

in the above example without getting an error?**

String operations

-----------------

A few basic string operations are presented here. 

**String concatenation**

String concatenation is done by simple addition of two strings.

::

  >>> x = 'Hello'

  >>> y = ' Python'

  >>> print x+y

  Hello Python

*Try this yourself:*

::

  >>> somenum = 13

  >>> print x+somenum

The problem with the above example is that here a string variable and an integer

variable are trying to be concantenated. To obtain the desired result from the 

above example the str(), repr() and the `` can be used.

**str()** simply converts a value to a string in a reasonable form.

**repr()** creates a string that is a representation of the value.

The difference can be seen in the example shown below:

::

  >>> str(1000000000000000000000000000000000000000000000000L)

  '1000000000000000000000000000000000000000000000000'

  >>> repr(1000000000000000000000000000000000000000000000000L)

  '1000000000000000000000000000000000000000000000000L'

It can be observed that the 'L' in the long value shown was omitted by str(), 

whereas repr() converted that into a string too. An alternative way of using 

repr(value) is ```value```. 

A few more examples:

::

  >>> x = "Let's go \nto Pycon"

  >>> print x

  Let's go 

  to Pycon

In the above example, notice that the '\n'(newline) character is formatted and 

the string is printed on two lines. The strings discussed until now were normal 

strings. Other than these there are two other types of strings namely, raw strings

and unicode strings.